Carbon-based fuels are used as combustion fuels for steam boilers. The boilers make steam to drive steam turbines in power generation plants. The turning of the steam turbines produces power. Conventional steam boilers burn fuel in the presence of air to form steam that can be used to drive a steam turbine. Typical steam boiler fuels include natural gas, petroleum, char, coke, heavy residual oil and coal (fossil fuels). When these carbon-based fuels are combusted in the presence of air they produce flue gas that is mostly nitrogen (N2), CO2 and lesser amounts of water vapor, with small quantities of material to be reduced, such as nitrogen oxides (NOx), sulfur oxides (SOx) and particulate matter.
NOx and SOx gases that are released into the atmosphere cause environmental pollution and are currently regulated. CO2 is considered a GHG and CO2 emissions are also proposed to be regulated. Recently the government has lowered the emission level allowed on NOx and SOx and has proposed reducing permitted CO2 emission levels.
There are varied CO2 capture techniques being proposed for steam boiler units, where CO2 is captured using various absorber scrubber systems (polar solvent based systems) that act on the cooler effluent flue gas stream. None of these CO2 absorber techniques for steam boiler units are believed to be capable of capturing CO2 with essentially no further atmospheric emissions.
Conventional steam boiler units have generally succeeded at producing electrical power, but require significant operating and capital expense to recover and capture CO2. Moreover, conventional steam boiler units generally use large amounts of energy to product electric power. In addition, CO2 capture with zero emissions has not been attempted successfully in conventional steam boiler units.
In conventional fossil fuel steam boiler units, the sulfur in combusted fuel forms SOx in the flue gas. The SOx is typically recovered using various techniques such as: (1) circulating fluidized bed (CFB) systems where calcium carbonate (CaCO3) is reacted with the SOx to form calcium sulfate (CaSO4), or (2) flue gas desulfurization (FGD) reactant systems using magnesium carbonate (MgCO3) or CaCO3 to recover the sulfur in the flue. These techniques usually recover 90% to 98% sulfur in fuel and represent significant operating and capital equipment costs.